1. Field of the Invention
The present invention relates to a manufacturing method of titanium compressor wheel used in automobile turbo charger and so on.
2. Description of the Related Art
For example, an automobile turbo charger has wheels 100, 200 on both end sides of a single rotary shaft 300 as shown in FIG. 2. Exhaust gas 250 discharged from a cylinder (not shown) of an engine is introduced to a wheel (turbine wheel) 200 and the turbine wheel 200 is rotated with exhaust gas pressure. The other wheel (compressor wheel) 100 is driven with that rotation force and the compressor wheel 100 compresses intake air 150 and supplies to an engine cylinder (not shown).
As shown in FIG. 1, this compressor wheel 100 comprises a front boss portion 101 and a rear boss portion 104 (see FIG. 13), fixed to a rotary shaft 300 (see FIG. 2) coaxially, a plurality of blades 102 extending radially from an inlet side 130 near the front boss portion 101 to an outlet side 140 outward in a radius direction and a disc-like core portion 103 extending outward in the radius direction at the rear boss portion 104 located on an opposite side to the inlet side 130 in an axial direction. The blade 102 is curved backward with respect to the rotation direction in order to compress intake air introduced in from the inlet side 130 and discharged from an outer peripheral portion of the outlet side 140. To intensify suction efficiency, long full blades 107 and short splitter blades 108 are arranged alternately and beginning end positions of both the blades 107, 108 are deviated. In the meantime, the blades 102 may be composed of only the full blades 107.
More specifically, the front boss portion 101 and the rear boss portion 104 have end faces 101a, 104a (see FIG. 11) and outer peripheral faces 101b, 104b (see FIG. 11). A shaft hole 109 goes through in the axial direction between the both end faces 101a and 104a. The core portion 103 has a hub face 103a serving as intake air flow face subsequent from the outer peripheral face 101b of the front boss portion 101, a back face 103b located on the back side of the hub face 103a (see FIG. 11) and an outer peripheral face 103c running along the outer edge of the core portion 103.
In the full blade 107 and splitter blade 108, pressure faces 107a, 108a are formed on upward sides in the rotation direction and negative pressure faces 107b, 108b are formed on downward sides in the rotation direction. Each blade 107, 108 is comprised of a leading edge 107c, 108c extending from the hub face 103a of the core portion 103 outward in the radius direction at the inlet side 130, a trailing edge 107d, 108d extending from the outer peripheral face 103c of the core portion 103 toward the inlet side 130 at the outlet side 140, and shroud 107e, 108e connecting the both edges 107c, 108c, 107d, 108d. 
Because the compressor wheel 100 (particularly blade 102) is constituted of complicated three-dimensional curved surfaces, generally it is manufactured by precision casting such as lost wax casting process or machining such as cutting controlled under numerical control of five axes or more.
In recent years, the compressor wheel 100 has been demanded for intensified performance such as higher rotation number and high compression ratio (high pressure ratio) in order to improve combustion efficiency of an engine, purifying exhaust gas and for compactness (downsizing) accompanied by intensified function of automobile and engine. Thus, instead of a conventional aluminum made compressor wheel, a titanium compressor wheel having a higher mechanical strength has been sometimes adopted and a manufacturing method of the titanium compressor wheel has been disclosed in Japanese Patent Application Laid-Open No. 2004-52754 and U.S. Pat. No. 6,588,485.
These documents have disclosed following technologies. First, a positive pattern (near net shape male pattern) of wax or the like is created by adding a pad to a portion in which an insert die (die) generates a non-pullable pattern such as undercut. After that, lost wax casting is carried out with the positive pattern (near net shape) having the pad as a basic pattern. Further, only the pad portion of a cast titanium product (near net shape) is removed by cutting or the like and as a consequence, a titanium compressor wheel (net shape) of a complete pattern (final shape) is obtained. Therefore, according to this manufacturing method, manufacturing cost required for cutting and the like can be suppressed.
However, because according to the disclosed technology, only the portion having the pad of that cast titanium product is cut and a portion having no pad of the cast titanium product (that is, a portion in which the insert die can be pulled out, generating no undercut) is not cut, a probability that the non-cut portion of the cast titanium product (pullable portion) may not satisfy a predetermined dimensional allowance due to thermal stress or the like generated at the time of casting is increased. Accompanied by this dimensional deviation, dynamic balance is likely to deteriorate and often it takes a long time to correct rotation balance after a completion. Further, often a completed product becomes defective because it does not fall under such a dimensional allowance thereby reducing yield of products. In the meantime, if a compressor wheel is produced from a lump of titanium only by mechanical processing such as cutting, manufacturing cost increases because it is harder than aluminum, accompanying a difficulty in its processing.